If you want to search the Library of Congress catalogue via Z39.50, you can do that here. What exactly is Z39.50? "Despite certain nominative similarities, it's not a robot from that other blockbuster of the summer, Star Wars: The Phantom Menace, but rather the cuddly and approachable name for an important standard of relevance to many working with information resources in a distributed environment", says Paul Miller in his article for Ariadne. You'll find a lot of information on the National Information Standards Organization's Z39.50 resource page as well as on the International Standard Maintenance Agency's website.